Crab Comet conversion

Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Tue Sep 17, 2019 5:43 pm

Julian.
I'm not sure what the relevance is of the eccentric rod being parallel to the connecting rod?
The outer end of the return crank should be approximately at right angles to the wheel boss crank, allowing for any difference in slope of the cylinder and the eccentric rod at mid swing. On the Crab, the end of the return crank described a 1' - 11/2" diameter circle. Unfortunately, this geometry will only be achieved if the wheel crank throw and the return rod are the correct lengths.
Hope that helps.
Dave.

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Tue Sep 17, 2019 8:31 pm

Dave many thanks. I would have to ask an awful lot of questions to properly understand that...but I think or hope the way I had it set already was good enough really...? As far as I know the crank is the right length and the wheel crank throw is also correct.

This shows the parallel situation of eccentric and connecting rod though it's not a dead side on photo. But I think I see it's not a good enough way to set the throw. As it was I just guessed from photos and checked there wasn't too much movement for the valve gear to handle. But of course I want the movement to be an amount that's discernible at a distance so the visual effect is more important than exact accuracy!
Screenshot_2019-09-17-21-19-13.png


Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Wed Sep 18, 2019 12:00 am

Hi Julian.
At a guess, I'd say the return crank on your Crab looks about right. Just over 2 mm in advance of the axle at bottom dead centre would be spot on.
I agree with you that an impression of reasonable valve gear movement is more important than the exact amount of travel and I generally aim for about 2 mm on all locos irrespective of the actual scale offset, although I don't actually measure it, just by eye. In practice, the ideal travel is compromised by the relationship between expansion link and slide bar support bracket, as not all kit valve gears are truly to scale. Sometimes, you just have to aim for the best you can get, given the kit parts, although I have been known to alter rod lengths by splicing, to obtain a better geometry.
There goes my reputation for accurate modelling!
Dave.

davebradwell
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Re: Crab Comet conversion

Postby davebradwell » Wed Sep 18, 2019 10:30 am

The wheel seat is 2.5 mm long at most and less if you file off the protruding front boss, as I do, on outside cylinder engines to allow the prototype valve gear alignment. This doesn't leave much space for even a single groove as you're relying on leaving some plastic to locate the wheel square to the axle. Consider making a shallow groove right round with the corner of a Swiss file - 0.1 or so deep. I'd avoid a flat as it's not symmetrical. The clever part of using Loctite is to avoid getting it in the axleboxes or your jig as it can stay liquid for ages outside a joint.

If you want to get deeper into valve gears there's plenty of stuff on the model engineering websites. It's unfortunate that we usually get one length of return crank in a kit and choose from a selection of wheels which have different throws. A good credibility check is to put the engine on back centre and the angle of the expansion link should be such that the engine can be changed from forward to reverse without the valve moving and you seem to pass this. The position of the expansion link pivot is important and I drill this in the frames so the brackets can be aligned correctly with a rod through.

Your anchor link seems to reach an awkward angle - is the drop link a bit long? As DaveH says it's all about appearance.

DaveB

John Palmer
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Re: Crab Comet conversion

Postby John Palmer » Wed Sep 18, 2019 6:45 pm

davebradwell wrote:Your anchor link seems to reach an awkward angle - is the drop link a bit long? As DaveH says it's all about appearance.
I agree about the angle of the anchor/union link, but I suggest that the reason may lie in the combination lever either being too short, or its point of attachment to the valve spindle being set a little too low. In either case that's likely to be due to the design of the lever's etching, so it was probably a case of having to make the best of what you were given.

I can't see the significance of the wheel crank throw other than setting limits to the eccentricity of the circle described by the return crank's joint with the eccentric rod; is there something I am missing about this?

That Crab's fireman will be earning his crust the hard way, with his driver continuously running the engine in full forward gear! ;)

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Wed Sep 18, 2019 8:38 pm

Thanks both Daves you've answered my questions enough for me! Something I asked earlier in this thread was whether a straight line from the valve spindle in the cylinder should meet the pivot of the expansion link. If so the kit didn't really do that. The pivot was a bit high. I laboured quite a lot to lower it and get the separation of the expansion link support bracket from the footplate that is noticeable on the prototype, but it only really happened on the right side in the end, and it's not really particularly noticeable on the model. I faffed around quite a bit with the height of the three support brackets but one's also working against the fact the plastic model body is a bit too flexible without the screw that fixes the front of the footplate to the body below the chimney. (In fact I've only just glued this up for good with Araldite.) I altered the height of the valve spindle crosshead slidebar bracket to align with the spindle from the cylinder....very fiddly and unsatisfactory, it just about hangs together.

Yes the union link (thanks John for translating) is a bit naff. I think it's better on the right (see below). I did enough alterations of the kit without embarking on altering the valve gear! - I took it on trust. I wonder if the length of the combination lever allows for the joggle it has. There wasn't anything in the instructions about a joggle. I just copied photos. So that could be the reason it brings the droplink too high. I'm going to live with it though and get on with the next projects. I fully expect the V3 will have this kind of thing properly designed!!

John the Fireman may not have to work so hard when he's facing away from the audience! :P On the RHS the gear is notched up about half way to midgear :D Dreadful I know. But I contemplated having full reverse.... :o

I can't answer your question though other than the more the throw the more everything moves.


davebradwell
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Re: Crab Comet conversion

Postby davebradwell » Wed Sep 18, 2019 10:17 pm

I found a broadside photo and thought your combination lever about right - the bottom pin comes over the lower edge of the slidebar on front centre. There is no rule that places the expansion link pivot on the centreline of the valve but that does seem to be the case here and your GA should confirm it. The tail comes on the centre of the cylinder bore, however but the expansion link is forced to be smaller than scale as the original goes behind the valence at the top which the model can't do. Just a difficult prototype.

The V3 had it's own difficulties with most of the expansion link and its bracket inside the splasher. An early attempt at design using the plastic body led to unacceptable distortion of the motion due to the over-thick platform compressing all that lay below. It's easier to sort these things out on paper first so I ended up etching the body, too. Yes, the joggle on the expansion link is specified.

DaveB

Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Thu Sep 19, 2019 11:01 am

Julian.
I've just compared my Comet Crab valvegear against the drawing in the Wild Swan profile book. The length of the bottom section of the combination lever, from valve spindle to the anchor link is pretty close, even with a slight joggle - 2' 8 1/2" vs about 11 mm . However, the cross head drop link is a bit over scale - should be 8 1/2" (just under 3 mm, but is actually about 4 mm. Perhaps I didn't put quite enough joggle in that. It's not a huge error, but does affect the angle of the anchor link, especially at the ends of the travel. If you have a more abrupt joggle in the combination lever than mine, that would also shorten the lever and make the anchor link more inclined up at its front end. (Note: On the published drawing, the dimensions are in such small font, that even with my magnifying lamp, I wouldn't swear I've read them correctly!)
Dave.

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Thu Sep 19, 2019 7:36 pm

Thanks so much both Daves. As a matter of er...interest?.... I didn't really know how to interpret a GA but had the drawing on the Bachmann box. I did file a slot into the LHS valence for the top of the expansion link. It wasn't needed on the RHS where I managed to get the bracket correctly slightly lower.

But it was all rather hopeless - the expansion link's characteristic and rather outrageous length is quite a feature of the prototype (maybe the reason for their nickname?) and not represented in either the kit nor in the Bachmann 00 version so these efforts were like whistling in the wind. Generally it's all a bit horrific and my lesson is not to buy a less than top quality (eg Bradwell :thumb :) ) kit. Effectively scratch building Walschaerts from a drawing does not appeal as a way of spending my life! - and there are plenty of less complex locos that are equally good motive power waiting to be built.

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Allan Goodwillie
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Re: Crab Comet conversion

Postby Allan Goodwillie » Thu Sep 19, 2019 9:31 pm

Ah Julian :(

we all get these feelings about our models when we get to this point of finishing, we all tend to overlook all the things we have managed to get right and yet there is nearly always a niggle somewhere and it is nearly always something about the kit that is just not quite there. Nearly every kit I have put together has had something about it!

I gave up building for others as it was frustrating as most just wanted the kit built and did not really care whether it was exactly right. I am no perfectionist and some time down the line you will realise you have done an excellent job and knowing your previous engines this will run well and give much pleasure.

I have been working on my engines going to Newcastle with Dubbieside in November so doing any repairs etc. The loco I have been working on is my Y9 built about 50 years ago. Under the magnifier light it seems so crude compared to more recently built engines. However it has always been one of the characters I have enjoyed having because it has that weary ancient look of one of these engines in its last years. The original paintwork has 50 years of patina which adds even more to the look of the engine. I rebuilt my J88 from the same period and did a complete repaint and although it is OK the patina of course disappeared and will take some years to rebuild.

I also remember the first time I built a loco with outside valve gear and the sense of accomplishment in getting it to operate - it was my A4 Kingfisher - now probably around 40+ years ago. Again there are probably one or two things I have managed to get wrong, but I no longer care about all that. I heard an interesting quote today from Celine Dion who said she did not strive to be the best of the best, but to be the best that she can be. The first part of that can lead to a form of madness, the second can lead to a lot of pleasure and is a manageable way to proceed in life.

Sitting listening to Bob Harris back on the radio tonight - so pleased he is back :D - so much of what I have built has been built against Bob's program on in the background and Mark Radcliff's folk program as well. Was watching the recent concert of the BBC Scottish Symphony Orchestra open air proms in Glasgow - was strange watching and for you not to be sitting there playing. Hope you are enjoying retirement, just be a little kinder to yourself when it comes to the engines.

Bob is playing Dylan at the moment - North Country Girl the version off National Skyline with Johnny Cash - a completely different version from the original folky one which he sang on one of his early LPs. As in music engines can be like that - different versions of the same thing, both in real life as in models. It is in the painting and weathering that they finally come to life and that has nothing to do with the odd fraction here and there. :)

Noticing your Compound in the background and it was taking good shape the last time I saw it - nice engines as well. It is good to see the two Dave's giving excellent and honest advice, one reason why I think the Forum is worth pursuing. We are never going to get it all right, but if we do things that stretch us and we have some success in building our models then that should be more than enough for any of us. It is only model railways.

Program is finished so am I :)
Allan

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Fri Sep 20, 2019 5:50 pm

:thumb what a nice post Allan! Yes my mantra is "let not the ideal be the enemy of the good" and I believe in the "good enough" philosophy too. That's why I'm not going to correct that rather high union link and why I couldn't contemplate filing my own expansion links.

When it's all said and done how the model moves is much more important than the slight shortcomings in appearance, and in the end why Walschaerts is quite a waste of time - a gracefully moving inside cylinder Pacific is almost as satisfactory as a fully detailed outside gear one, I think. Arguably more satisfactory I expect McIntosh fans would argue re his Carndean for example.

Yes it's the painting that will bring it to life and I've started by priming the valve gear red oxide while it's off the loco. Then I will build up the colour so as to hopefully make the moving gear which has all told taken two years as visible as is the unpainted version in the above videos - yet how to do it without it being gross?... hopefully you can be a guide. I'll post a picture of the reassembled model when it's done.

Not quite retired yet. I miss the Last Night of the Proms by choice whenever I can. Never have understood its popularity and in today's climate I find it pretty disturbing. :?
Last edited by Julian Roberts on Fri Sep 20, 2019 7:40 pm, edited 1 time in total.

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Le Corbusier
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Re: Crab Comet conversion

Postby Le Corbusier » Fri Sep 20, 2019 6:32 pm

from the view point of a simple MR 0-6-0 tank builder, I think your work looks 'bloody marvellous' Julian ...... just saying ;)
Tim Lee

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Allan Goodwillie
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Re: Crab Comet conversion

Postby Allan Goodwillie » Fri Sep 20, 2019 7:28 pm

Hi Julian,

Not sure if you have been following Tim's thread Julian - having a first go at a springy beam conversion using a Gibson profiled chassis as a starting point. Looking forward to seeing this loco developing as well. He has made a very good and practical start and was having to deal with spacings being slightly out. His solution should work OK.

It was the Glasgow prom by the way -a real mix without all the jingoistic nonsense, although a bit of pandering to Glasgow tastes.

Allan :)

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sat Oct 12, 2019 7:43 pm

Very tardily, Thank you to Tim for your kind words. And yes Allan the Glasgow Proms "Last Night" is much more Glaswegian than British, and much the better for it. Now I've had my last concert (a proper one!) as a full time player, though I'm continuing for the present with them as a freelancer.

It's taken an age to get the time, but now the Crab is put back together with a new pair of centre wheels. The problems I had there made me start a separate thread. viewtopic.php?f=20&t=6602 Here is the loco back working, and none the worse for the misadventure. In fact rather better possibly in terms of smooth slow running, to my surprise. Most of this test track is a pair of reverse 4ft curves by the way.

The return cranks were now both about three quarters of a full turn forward of the correct orientation when screwed down without shims. The main shim is now a 14BA washer, plus some 0.06mm shim IIRC. I wasn't thinking about it till after the cranks were glued down, but luckily the washers are just that bit big enough for the coupling rods to not ride over them, so they space the rods out from the wheel boss too. The photo from below shows the clearances. The main problem not addressed by the kit is the eccentric rod clearance from the return crank. On the left hand side the clearance is fag paper thin, remaining so on a right hand bend, even with the extra washer I put there. It is slightly bent but as it is'nae broke I'm not gonnae fix it. After several trials with Gibson steel screws and brass nuts I decided the Threadlock would be OK to fix the return crank if used for just the last turn.
20191012_122312.jpg

I painted the valve gear a mix of gloss dark blue and a reddy brown colour , a thin wash that covered it
20191012_121753.jpg

The next morning I used a glass fibre brush on some areas to further thin it down. Of course how it looks depends very much on the light. Exhibition lighting is not like the setting sun which lights up the wheels etc. Being overhead the area tends to look obscure, and that's why I want to bring it out more than is perhaps really realistic.
20191012_175229.jpg

By changing just one number the loco now becomes Scottish, based at Ayr, rather than English based at Fleetwood. The 4 needs a bit of toning down still.

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Allan Goodwillie
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Re: Crab Comet conversion

Postby Allan Goodwillie » Sun Oct 13, 2019 5:52 pm

Hi Julian, :)

Very nice indeed, your video shows some very nice running and control, including the drop in the rail. Looking forward to seeing the loco running on Calderside with a descent train.

Don't know if you have had a look at this film made by the American Army during the second world war.

https://www.youtube.com/watch?v=agznZBiK_Bs

I am not suggesting doing this on Calderside!

Well dne!

Allan :)

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sun Oct 13, 2019 7:23 pm

Haha Allan! That film might be taking Mike Sharman's ideas of challenging trackwork beyond their limits!


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